Strength and biaxial formability of cryo-rolled 2024 aluminium subject to concurrent recovery and precipitation

The precipitate-hardenable aluminium alloy 2024 has been processed by rolling to develop a fine microstructure. Four alloy conditions were tested; these included two rolling temperatures and two different ageing sequences. For all four conditions there was an ideal heat-treatment time at which there was a concurrent improvement in both strength and formability. Microstructural modeling has shown that this is the result of a small processing window in which the hardening due to precipitation is larger than the softening due to recovery, while the detrimental effects of particle coarsening on ductility have not yet developed. Cryo-rolling and room-temperature rolling produced materials with similar strengths, but cryo-rolling showed inferior formability. Natural ageing before rolling significantly decreased the formability compared to rolling in the supersaturated condition, and it is proposed that the solute clusters that develop during natural aging inhibit dynamic recovery and consequently increase the dislocation density that develops during rolling.